Resonance type optical deflecting devices proposed conventionally have the following features as compared with scanning optical systems using a rotary polygonal mirror such as a polygon mirror. That is, the optical deflecting device can be reduced in size considerably, the power consumption is slow, and theoretically there is no surface tilt of the mirror surface.
On the other hand, in the resonance type deflectors, since the deflection angle (displacement angle) of the mirror changes sinusoidally in principle, the angular speed is not constant. In order to correct this characteristic, techniques have been proposed in U.S. Pat. No. 4,859,846 and U.S. Patent Application Publication No. US2006/0152785.
In U.S. Pat. No. 4,859,846, a resonance type deflector having oscillation modes of a fundamental frequency and a frequency threefold the fundamental frequency is used to accomplish chopping-wave driving. FIG. 14 shows a micromirror which realizes approximately chopping-wave driving. An optical deflecting device 12 comprises oscillators 14 and 16, torsion springs 18 and 20, driving members 23 and 50, detecting members 15 and 32, and a control circuit 30. This micromirror has a fundamental resonance frequency and a resonance frequency approximately threefold the fundamental frequency. It drives based on a synthesized frequency of the fundamental frequency and the threefold frequency. As a result, the oscillator 14 having a mirror surface is driven by chopping-wave driving, and the deflection angle thereof realized optical deflection with less change of the angular speed as compared with sinusoidal driving. On this occasion, the oscillation of the oscillator 14 is detected by the detecting members 15 and 32, and a necessary driving signal for the chopping wave is generated by the control circuit 30. The micromirror is then driven by the driving members 23 and 50.
On the other hand, U.S. Patent Application Publication No. US2006/0152785 discloses a microoscillator in which a system comprised of a plurality of torsion springs and a plurality of movable elements has a plurality of separated natural oscillation modes. In this microoscillator, within the separate natural oscillation modes, there are a reference oscillation mode which is the natural oscillation mode of the reference frequency, and an even-number-multiple oscillation mode which is the natural oscillation mode of a frequency approximately n-fold the reference frequency where n is an even number. In U.S. Patent Application Publication No. US2006/0152785, the microoscillator is oscillated based on these oscillation modes, whereby sawtooth-wave driving is realized.
Furthermore, in Japanese Laid-Open Patent Application No. 2005-292627, in order to detect the scan position of a light beam deflected by a deflection mirror being driven based on a sinusoidal wave, the time moment whereat the light beam being scanningly deflected passes a predetermined position is detected by means of an optical sensor, whereby the deflection mirror is controlled using the time moment.
Although the oscillator devices of U.S. Pat. No. 4,859,846 and U.S. Patent Application Publication No. US2006/0152785 mentioned above have realized chopping-wave driving and sawtooth wave driving, since the resonance frequency of the oscillator is different due to the manufacturing error or operating environment, it is necessary to detect the resonance frequency of the oscillator at the time of driving.
The oscillation state of the oscillator can be detected by, as disclosed in Japanese Laid-Open Patent Application No. 2005-292627, providing a photodetector near the scan end of the light beam (scanning light) deflected by the oscillator device, and based on the timing when the scanning light passes this photodetector.
For example, if the resonance frequency of an oscillation system having a plurality of oscillation modes (a reference oscillation mode and an oscillation mode integral-multiple of the reference mode) such as disclosed in U.S. Pat. No. 4,859,846 and U.S. Patent Application Publication No. US2006/0152785 is going to be detected, the oscillator is driven with the respective oscillation modes and the state of oscillation of the oscillator at that time is detected.
If the state of oscillation is detected using a photodetector provided in the vicinity of the scan end, when the oscillator is driven based on the reference oscillation mode, the scan range of the scanning light can be comparatively broadened, such that the state of oscillation of the oscillator can be detected with the photodetector provided near the scan end.
However, when the oscillator was driven only by the oscillation mode of integral-number multiple, the scan range of the scanning light becomes narrower as compared with the case where it is driven by the reference oscillation mode. Therefore, it is difficult to detect the state of oscillation of the oscillator by use of the photodetector disposed near the scan end. Thus, it is difficult to detect the resonance frequency of the oscillation mode of the integral-number multiple of the oscillation system.